This invention has to do with locks, and relates especially to locks that require two keys for their operation, as is typically true, for example, in locks for safe deposit boxes and similar applications.
Locks of that general type ordinarily require a correspondingly large number of parts, making them relatively expensive to manufacture and service. In particular, if each key drives a distinct and independent set of tumblers the number of moving parts in the lock may approach twice that required in a single-key lock.
Dual key locks have been proposed, as in U.S. Pat. No. 3,127,759 to Roy T. Ellis, for example, having a single stack of floating tumblers which are engaged by the two keys at spaced positions along their length. However, the tumblers of Ellis' single stack are of complex shape and require numerous auxiliary levers and secondary tumblers for their operation. Moreover, the gate positions on each tumbler's primary working edge must take account of the bit height of both the keys. Thus, if the key combination is to be changed by replacing one set of tumblers by a set corresponding to a new pair of keys, a very large number of different tumbler forms must be kept on hand.
The invention described in the above identified parent application provides dual key locks which combine remarkable simplicity of basic form with improved accuracy of operation. The locks of that invention typically require no more parts, aside from the obvious need for two key noses or posts, than the most rudimentary of single key locks having the same number of teeth per key. The structure of the invention permits the keys to position the lock tumblers with such accuracy that it becomes feasible to provide an increased number of bit heights within a given range of radial key dimensions.
Those and other advantages of the invention are typically attained by employing a single stack of elongated tumblers each of which has two gated transverse blocking edges adapted to cooperate with respective fence lugs. The tumblers are laterally positioned by dual keys that engage the tumblers directly opposite the respective blocking edges. That simple configuration has been found to permit each key to act accurately and reliably to bring the corresponding gates into alignment to release the corresponding fence lug essentially independently of the action of the other key.
That independence of function of the two keys further permits the tumblers to be designed with such symmetry that each one can be installed in the lock in two, or preferably in four, alternative orientations, each such orientation corresponding to a different combination of bit heights on the two keys. Since each tumbler can provide four, say, different key combination, the number of different tumbler configurations required to make up any desired number of key combinations is typically reduced by a factor of four. The cost and labor of making key changes are thus significantly reduced.